If one had a nega of the hair or beard the size of a split bean and it spread over all his head, he is ritually clean. Rabbi Yehuda says that the head and the beard do not block one another but Rabbi Shimon says that they do. Rabbi Shimon said that, logically, since the skin of the face and the skin of the body – which are not contiguous – block one another, it only makes sense that the head and the beard – which are contiguous – should also block one another. The head and the beard don’t combine, nor does spreading from one to the other have any effect. The beard is from the joint of the jaw to the bulge in the trachea.
A bald scalp or forehead is rendered impure for two weeks by two signs: healthy flesh and spreading. Baldness is as follows: if one ate a depilatory or smeared himself with it, or if he had a wound from which hair can no longer grow. A bald scalp is from the crown of the head sloping back to the knob of the neck. A bald forehead is from the crown of the head sloping forwards to the hairline. A bald scalp and forehead do not combine, nor does spreading from one to the other have any effect. Rabbi Yehuda says that if there’s hair between them, they don’t combine but if there’s no hair between them, then they do combine.